Add documentation

services/matching-service/README.md

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+# User Service
+
+## Pre-requisites:
+1. Install dependencies using `yarn install`
+2. Run `yarn prisma generate`
+3. `yarn workspace matching-service run dev`
+4. Look at [index.html](./src/routes/index.html) to see how to use the API
+
+## API:
+**Note:** All API endpoints are prefixed with `/api/matching-service`
+
+Matching service **DOES NOT** use REST API. It uses socket.io to communicate with the client.
+The flow is:
+
+1. Client connects to /api/matching-service using socket.io and provides their username
+2. Client can send messages to the server using socket.io as such:
+```js
+const username = "alice";
+const socket = io('http://localhost:5002/api/matching-service', {query: `username=${username}`});
+const difficulties = ["easy", "medium", "hard"];
+const programmingLanguage = "java";
+socket.emit('lookingForMatch', difficulties, programmingLanguage); // Looks for a new match
+socket.emit('cancelLooking'); // Stop looking for a new match
+socket.emit('leaveMatch'); // Leave the current match (only if you are in a match)
+socket.emit('sendMatchMessage', "Hello World!"); // Send a message to the other user in the match
+```
+3. Client should listen for messages from the server as such:
+```js
+socket.on('matchFound', (match) => {
+    console.log(match); // See Match type in prisma schema
+});
+socket.on('matchNotFound', () => {
+    console.log("Probably timed out, can't find a match");
+});
+socket.on('matchLeft', () => {
+    console.log("Your match left the match"); // Other user emitted "leaveMatch"
+});
+socket.on("receiveMessage", (message) => {
+    console.log(message); // Message from the other user or the server
+});
+socket.on("error", (error) => {
+    console.log(error); // Error from the server
+});
+```
+4. When a client wants to look for a match, the match request is placed in an in-memory queue. The server will try to match the user with another user in the queue. If a match is found, both users will be notified and will join a room. The server will then send a message to the client with the match information. If a match is not found after a specified time (60 seconds), the client will be notified and the request will be removed from the queue.
+
+Notes:
+
+We use socket.io instead of traditional HTTP REST API
+Matched users will join a room
+We use EventEmitters to notify the other waiting user of a new match
+
+
+Extensions:
+
+Store the waiting users on a persistent queue in case the service dies (Use redis possibly)
+Remove REST API code